Fractionalized Stimulation Pulses in an Implantable Stimulator Device

ABSTRACT

A method for configuring stimulation pulses in an implantable stimulator device having a plurality of electrodes is disclosed, which method is particularly useful in adjusting the electrodes by current steering during initialization of the device. In one aspect, a set of ideal pulses for patient therapy is determined, in which at least two of the ideal pulses are of the same polarity and are intended to be simultaneous applied to corresponding electrodes on the implantable stimulator device during an initial duration. These pulses are reconstructed into fractionalized pulses, each comprised of pulse portions. The fractionalized pulses are applied to the corresponding electrodes on the device during a final duration, but the pulse portions of the fractionalized pulses are not simultaneously applied during the final duration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 15/210,726,filed Jul. 14, 2016, which is a continuation of U.S. patent applicationSer. No. 14/329,608, filed Jul. 11, 2014 (now U.S. Pat. No. 9,393,423),which is a continuation of U.S. patent application Ser. No. 12/824,663,filed Jun. 28, 2010 (now U.S. Pat. No. 8,812,131), which is acontinuation of U.S. Patent application Ser. No. 12/121,281, filed May15, 2008 (now U.S. Pat. No. 7,890,182). These applications areincorporated herein by reference in their entireties and priority isclaimed to them.

FIELD OF THE INVENTION

The present invention relates to therapeutic electrical stimulationsystems and methods and, more specifically, relates to adjustingelectrodes of an implantable stimulator device.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a spinal cord stimulation system, such as thatdisclosed in U.S. Pat. No. 6,516,227, which is incorporated herein byreference in its entirety.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. As shown in FIGS. 1A, 1B, 2A,and 2B, a Spinal Cord Stimulation (SCS) system typically includes anImplantable Pulse Generator (IPG) or Radio-Frequency (RF) transmitterand receiver 100 (collectively, “IPGs”), at least one electrode lead 102and/or 104 having a plurality of electrodes 106, and, optionally, atleast one electrode lead extension 120. The electrodes 106 are arrangedin a desired pattern and spacing on the lead(s) 102, 104 to create anelectrode array 110. Wires 112, 114 within one or more leads(s) 102, 104connect each electrode 106 in the array 110 with appropriate currentsource/sink circuitry in the IPG 100.

In an SCS application, the electrodes lead(s) 102, 104 with theelectrodes 106 are typically implanted along the spinal cord 19 (FIG.2B), and the IPG 100 generates electrical pulses that are deliveredthrough the electrodes 106 to the nerve fibers within the spinal column.The IPG 100 body itself is normally implanted in a subcutaneous pocket,for example, in the patient's buttocks or abdomen. The electrode lead(s)102, 104 exit the spinal column and generally attach to one or moreelectrode lead extensions 120 (FIG. 2), which in turn are typicallytunneled around the torso of the patient to the subcutaneous pocketwhere the IPG 100 is implanted. Alternatively, if the distance betweenthe lead(s) 102, 104 and the IPG 100 is short, the electrode lead(s)102, 104 may directly connect with the IPG 100 without lead extensions120. For examples of other SCS systems and other stimulation system, seeU.S. Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated byreference in their entireties. Of course, an IPG 100 is an active devicerequiring energy for operation, which may be provided by an implantedbattery or an external power source.

Precise placement of the lead(s) 102, 104 relative to the target nervesis important for achieving a satisfactory physiological response, andfor keeping stimulation thresholds low to conserve battery power. Aconventional lead implantation procedure commonly places the leads 102,104 parallel to the spinal cord column 19 at or near the physiologicalmidline 91, as is shown in FIGS. 3A and 3B. More particularly, and asbest shown in the cross section of FIG. 3B, the electrode leads 102, 104are placed directly on the dura mater 51 within the epidural space 70.(Cerebro-spinal fluid 72 is between the electrode array 110 and thewhite matter 52 of the spinal cord 19. Dorsal root nerves 50 are shownemanating from grey matter 53). When the leads 102, 104 are placed onopposite sides of the physiological midline 91 as shown, additionalflexibility is provided in the ability to recruit (i.e., stimulate)nerves in the dorsal column, and to treat symptoms manifesting on eitherthe left or right sides of the patient's body.

In addition to precise placement of the electrode array, properselection of the electrodes, i.e., determining which of the electrodes106 in the array should be active in a given patient, is critical forachieving effective stimulation therapy. However, because of theuncertainties of the distances of the electrodes from the neural target,the unknown nature of the specific conductive environment in which theelectrode is placed, etc., it generally cannot be known in advance andwith precision which combination of active electrodes will be perceivedby a patient as providing optimal therapy. As a result, patient therapygenerally requires that various electrode combinations be tried andfeedback received from the patient as to which of the combinations feelsmost effective from a qualitative standpoint.

Various electrode combinations and other stimulation parameters can betried during initialization by programming the IPG 100 using an externalwireless clinician or hand-held controller. (Details concerning suchcontrollers can be found in U.S. Patent Publication 2007/0239228,published Oct. 11, 2007, which is assigned to the present applicationand which is incorporated herein by reference in its entirety). Forexample, and as best visualized in FIG. 3A, the IPG 100 can beprogrammed such that electrode E1 comprises an anode (source ofcurrent), while E2 comprises a cathode (sink of current). Or, the IPG100 can be programmed such that electrode E1 comprises an anode, whileE9 comprises a cathode. Alternatively, more than one electrode can beused in both the sourcing and sinking of current. For example, electrodeE1 could comprise an anode, while both E2 and E9 can comprise cathodes.The amount of current sourced or sunk can also be programmed into theIPG 100. Thus, in the last example, electrode E1 could sink 5 mA, whileelectrode E2 sources 4 mA and electrode E9 sources 1 mA. The frequencyof electrode stimulation pulses, as well as the pulsewidth or durationof such stimulation pulses, is also programmable. As disclosed in theincorporated '228 Publication, the time of no stimulation between pulsesis preferably greater than or equal to 3 milliseconds. See '228Publication, ¶65. As disclosed in U.S. Pat. No. 6,516,227 (incorporatedabove), control logic in the IPG 100 provides stimulation parameters tocurrent source and sink circuitry, which convert the receivedstimulation parameters into a current that is sourced to or sunk from anelectrode. See '227 Patent, col. 21, 1. 48-col. 23, 1. 47.

Ultimately, which electrodes are activated by the IPG 100, and thepolarities (cathode v. anode), magnitudes (amount of current), andfrequencies of those activated electrodes, are based largely on patientfeedback during IPG initialization as noted earlier. Thus, the patient,perhaps assisted by a clinician, will experiment with the variouselectrode settings, and will report relative levels of comfort andtherapeutic effectiveness to arrive at electrode settings that are bestfor a given patient's therapy.

In the prior art, patients and/or clinicians used a technique called“field steering” or “current steering” to try and simplify the iterativeprocess for determining a patient's optimal electrode settings duringinitialization of the IPG. See, e.g., U.S. Pat. No. 6,909,917, which isincorporated herein by reference in its entirety. In current steering,the current sourced or sunk by the electrodes is gradually redistributedby the patient or clinician to different electrodes using a singlestimulation timing channel. Such steering can be facilitated using somesort of user interface associated with the external controller, such asa joystick or other directional device. Simple examples of currentsteering are shown in FIGS. 4A, 4B, and 5. Starting first with FIG. 4A,assume that the IPG 100 has an initial condition, namely that electrodeE1 has been programmed to sink 10 mA of current, while electrode E3 hasbeen programmed to source 10 mA of current. This initial condition mightbe arrived at after some degree of experimentation, and might be acondition at which the patient is feeling a relatively good response,but a response which has not yet been fully optimized.

In an attempt at further optimization, current steering can commencefrom these initial conditions. Assume that optimization by currentsteering will ultimately arrive at the final condition of FIG. 4B. Asshown, this final condition sinks 10 mA at electrode E2. Thus, duringcurrent steering, 10 mA of sink current is moved from E1 (the initialcondition) to E2 (the final condition). To do this, electrode E1 isselected and the current sunk from that electrode is moved downward, forexample, by clicking downward on the controller's joystick. As shown inFIG. 5, this moves some increment of sinking current (as illustrated, a2 mA increment) from electrode E1 to electrode E2, such that E1 nowsinks 8 mA and E2 sinks 2 mA. Another downward click moves another 2 mA,so that now E1 sinks 6 mA and E2 sinks 4 mA, etc., until the full 10 mAis moved to E2 as per the final condition.

Gradual steering of the current in increments is generally consideredadvisable to safeguard against abrupt changes of the stimulation fieldwhich may be uncomfortable or dangerous for the patient. Abrupt shiftingof the entirety of the current from one electrode to another could haveunforeseen and undesirable effects. Different nerves are affected bysuch a change in electrode activation, and it is not necessarily knownhow moving a full allotment of current would affect those nerves. If thecurrent when applied to the new electrodes (e.g., from E1 to E2) is toolow (i.e., sub-threshold), no clinical response would be noticed, evenif the electrodes were ultimately suitable choices. If the current istoo high (i.e., supra-threshold), the result might be painful (ordangerous) for the patient. Accordingly, incremental movement of thecurrent is considered a good approach.

However, the illustrated current steering approach requires twodifferent electrodes (e.g., E1 and E2) to simultaneously act as currentsinks during the intermediate steering steps. This can be animplementation problem in IPG architectures that don't allow thesimultaneous selection of two or more electrodes to act as the source orsink. For example, some simpler IPG architectures may provide only asingle current source circuit and a single current sink circuit, whichcircuits can only be coupled to one electrode at a time. Because sucharchitectures will not support simultaneous activation of two or moreelectrodes as sinks or sources, the current steering approach of FIG. 5can't be used.

Other current steering approaches provide additional complexities. Forexample, the current steering approach illustrated in FIG. 6 isdisclosed in U.S. Patent Publication 2007/0239228, which wasincorporated by reference above. In this approach, steering of thecurrent from one electrode to another occurs by establishing the steeredcurrent in a second timing channel. (Because the operation of timingchannels are explained in detail in the '228 publication, they are notfurther explained here). Thus, and as shown, current in the transferringelectrode (E1) is initially established in a first timing channel ‘A.’As the current is incrementally steered to receiving electrode E2, thatsteered current forms in a second timing channel ‘B,’ such that thepulses in timing channel A and B are non-overlapping. The result afterseveral incremental transfers of current is the final condition in whichthe sink current resides entirely with electrode E2 is in the secondtiming channel B.

This approach of the '228 publication thus requires IPG hardware andsoftware necessary to support different timing channels. Not all IPGswill have such hardware or software, and so will be unable to benefitfrom the current steering technique of FIG. 6. Even in those IPGs thatcan support multiple timing channels, such a current steering techniqueis relatively complex, and is potentially limited. For example, althoughnot shown in FIG. 6, one skilled in the art will understand that thepulses must generally be followed by either a passive or active currentrecovery period. Because pulses in the next timing channel cannot beexecuted until currently recovery of the pulses in the preceding timingchannel is completed, the ability to use the '228 publication's currentsteering technique is not guaranteed. For example, if the stimulationpulses are of long duration or of a high frequency, there may simply notbe enough time in which to interleave the pulses in the two timingchannels, especially when current recovery periods are considered.

Accordingly, what is needed is an improved method for optimizingelectrode activation during the set up of an implantable stimulatordevice, and this disclosure provides embodiments of such a solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an electrode array and the manner in which it iscoupled to the implantable stimulator device in a SCS.

FIGS. 2A and 2B show a placement of the percutaneous lead for spinalcord stimulation with an in-line electrode array inserted alongside thespinal cord in the epidural space, in close proximity to the dura mater.

FIG. 3A and 3B show placement of two in-line electrode arrays on theleft and right sides of the physiological midline of the spinal cord,respectively, in a perspective view and in cross-section.

FIGS. 4A, 4B and 5 show an electrode current steering technique of theprior art.

FIG. 6 shows another electrode current steering technique of the priorart.

FIGS. 7A and 7B show how ideally simultaneous pulses can bereconstructed as non-simultaneous fractionalized pulse portions inaccordance with an embodiment of the disclosed technique.

FIGS. 8A and 8B show how the fractionalized pulse portions can be usedin a current steering application.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, the invention is not solimited. Rather, the invention may be used with any type of implantablemedical device system. For example, the present invention may be used aspart of a system employing an implantable sensor, an implantable pump, apacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator,a stimulator configured to produce coordinated limb movement, a corticaland deep brain stimulator, or in any other neural stimulator configuredto treat any of a variety of conditions.

A method for configuring stimulation pulses in an implantable stimulatordevice having a plurality of electrodes is disclosed, which method isparticularly useful in adjusting the electrodes by current steeringduring initialization of the device. In one aspect, a set of idealpulses for patient therapy is determined, in which at least two of theideal pulses are of the same polarity and are intended to besimultaneously applied to corresponding electrodes on the implantablestimulator device during an initial duration. These pulses arereconstructed into fractionalized pulses, each comprised of pulseportions. The fractionalized pulses are applied to the correspondingelectrodes on the device during a final duration, but the pulse portionsof the fractionalized pulses are not simultaneously applied during thefinal duration.

An improved current steering technique for an implantable stimulatordevice is illustrated in FIGS. 8A and 8B. However, before discussion ofthat technique, a more fundamental understanding of the technologicaland biological aspects of the technique are illustrated in FIGS. 7A and7B.

FIG. 7A illustrates an intermediary set of pulses as might be desiredduring current steering. As shown, electrodes E1 and E2 are desired tobe simultaneously asserted as pulses 201 a and 202 a, each providing a 5mA sink. This condition of simultaneity could be encountered whentransferring sink current from E1 to E2, as was illustrated earlier.

The actual implementation of such idealized pulses according to anaspect of the invention comprises a reconstruction of these ideal pulses201 a and 202 a as fractionalized pulses 201 b and 202 b which are notsimultaneous. As can be seen in the magnified illustration at the bottomof FIG. 7A, only one fractionalized pulse portion 205 or 206 is assertedat any given time. As such, the fractionalized pulse portions 205 and206 are interleaved.

The frequency of the fractionalized pulse portions 205, 206 in theillustrated example equals 1/t_(P), where t_(P) comprises the pulseportion period. Because the pulse portion period t_(P) is generally muchshorter than the duration of the ideal pulses, t_(D), there wouldtypically be many fractional pulse portions 205 or 206 occurring withinduration t_(D), although only a few such portions are shown in FIG. 7Afor ease of illustration.

Stimulation using fractionalized pulses 201 b and 202 b causes recruitedneurons to react to the pulse portions 205 and 206 in an additivemanner. For example, for a depolarizing sequence, the transmembranepotential will slowly depolarize on average with each additional pulseportion. The sequence of pulse portions 205 and 206 takes advantage ofthe non-linear membrane dynamics which tend to move a recruited neurontowards depolarization. In particular, short pulse portions will tend toopen the “m” gates of the sodium channels. As the gates open, the cellmembrane will tend to depolarize slightly more. A combination of pulseportions can then depolarize the membrane enough until, as with theideal pulses 201 a and 202 a, an action potential is generated in therecruited neurons. In other words, the nerves recruited by theelectrodes E1 and E2 will receive effective therapy even though thefractionalized pulses 201 b and 202 b are interrupted andnon-simultaneous unlike their ideal counterparts 201 a and 202 a.

The pulse portion period, t_(P) (frequency 1/t_(P)), may be kept lowerthan the chronaxie time, which is approximately 100 to 150 microsecondsor so (i.e., 10000 to 6666.7 Hz frequency). However t_(P) can alsoexceed the chronaxie time, although in such an application higherenergies (e.g., pulse portion amplitudes) might be required as explainedfurther below. However, effectiveness in therapy, even with increasingenergies, would be expected to diminish when t_(P) exceeds 500microseconds or so (i.e., lower than 2000 Hz).

In the illustrated example, the fractionalized pulse portions 205 and206 have a duty cycle of approximately 50%, such that only one pulseportion 205 or 206 from fractionalized pulses 201 b and 202 b areasserted at any given time. Note also that the amplitude of those pulseportions 205, 206 (−10 mA) are twice what is called for in thecorresponding ideal pulses 201 a and 201 b (−5 mA). This amplituderelates to the duty cycle of the pulse portions, and stems from therecognition that the total amount of injected charge remains animportant first-order variable in effective patient therapy. Thus, whenone compares the ideal pulses 201 a and 202 a with their fractionizedactual counterparts 202 a and 202 b, the amount of charge (i.e., thearea under their curves) is same. So, if the duty cycle of thefractionalized portions are 50%, an amplitude of twice would beindicated; if the duty cycle is 33.3% (as might occur should threeelectrodes need to act as either a source or sink at one time), threetimes the amplitude would be indicated, etc.

However, it is not strictly required that the amount of charge in theideal and fractionalized pulses be equal, and in a given applicationamplitudes of the fractionalized pulse portions 205 and 206 may need tobe adjusted to provide slightly more or less charge than the injectedcharge of the ideal pulses 201 a and 202 a. In one example, and asalluded to above, longer pulse portion periods might require higheramounts of charge than are represented by their ideal counterparts. Forexample, assuming a 50% duty cycle and a pulse portion period t_(P)slightly above the chronaxie time, the amplitude of the fractionalizedpulse portions 205 and 206 may be higher than double (e.g., 2.1 times)the amplitude of the corresponding ideal pulses 201 a and 202 a,resulting in a higher amount of charge. If t_(P) is made even larger,than the amplitude of the fractionalized pulses portions could increaseeven further (e.g., to 2.2 times), etc.

Because of the increase in amplitude of the fractionalized pulseportions 205, 206, the current generation circuitry in the IPG 100 mustbe capable of sustaining higher compliance voltages. See U.S. PublishedPatent Application 2007/0097719, published May 3, 2007, which isincorporated herein in its entirety, for a further discussion ofcompliance voltage generation in IPGs.

FIG. 7B illustrates another way in which simultaneous pulses can bereconstructed in accordance with the invention. As with FIG. 7A, theideal pulses 201 a and 201 b are fractionalized and interleaved as shownat 201 c and 202 c. However, the fractionalized pulse portions 205′ and206′ have the same amplitude (−5 mA) as do their ideal pulsecounterparts, but the fractionalized pulses 201 c and 202 c have aduration t_(D)' of twice the duration t_(D) of the ideal pulses. Theresult, as with FIG. 7A, is ideal and fractionalized pulses that arecomprised of approximately the same amount of charge. While thereconstruction method of FIG. 7B does modify the duration of the idealpulses (i.e., from t_(D) to t_(D)′), such pulse duration modificationonly affects patient therapy as a second-order variable; themore-important first-order variable of total charge remains essentiallyunchanged, and so patient therapy is not significantly impacted by thechange in duration. Of course, assuming that the amount of charge iskept approximately the same, other durations, both longer and shorterthan the duration of the ideal pulses, can be used in the actualfractionalized pulses. The two-fold duration increase shown in FIG. 7Bis therefore merely exemplary.

The system may use both techniques—higher pulse amplitude (FIG. 7A) orhigher pulse duration (FIG. 7B)—as convenient. For example, the logic inthe IPG 100 may choose an appropriate pulse fractionalization strategythat saves energy. Or, the logic in the IPG 100 may choosefractionalization parameters to prevent saturation of the output—e.g.,if the amplitude has been maximized then the pulse width or duration isincreased, etc.

There are significant benefits to reconstructing the ideal pulses asfractionalized pulses as shown in FIGS. 7A and 7B. Even though thepatient's nerves biologically sense simultaneous stimulation, thereality is that no more than one electrode is truly active as a sourceor sink at any given time, given the interleaved fractionalized pulseportions 205 and 206. Therefore, this technique, and the steeringtechnique described subsequently in FIGS. 8A and 8B, can be implementedin IPGs having simpler architectures in which source or sink circuitryis coupleable to only a single electrode at a time.

In an actual implementation, there would be some set-up time necessaryto switch current sink circuitry from E1 to E2, and so the duty cyclesof the fractionalized pulse portions 205 and 206 may be less than anideal 50% for example. However, such set-up time would be relativelyshort compared to the pulse portion period t_(P), and so such set-uptime is negligible and therefore not illustrated in the figures. Forexample, it may take only a few 0.1 of a microsecond to switch thecurrent from one electrode to another, i.e., from a pulse portion 205 toa pulse portion 206. However, one skilled in the art will realize thatthe transition times and other non-idealities will mean that the actualcharge of the fractionalized pulses may only approximate the chargespecified by the ideal pulses.

FIGS. 8A and 8B illustrate how the reconfigured fractionalized pulsescan be utilized in an improved current steering scheme. As with FIGS. 5and 6, FIGS. 8A and 8B illustrate the simple example of graduallysteering 10 mA of sink current from electrode E1 to electrode E2. Theinitial condition at the top of FIG. 8A is defined in a single timingchannel, which timing channel specifies the amplitude, duration, andfrequency of the ideal stimulation pulses. Starting from that initialcondition, a user (patient or clinician) selects to move an increment(e.g., 2 mA) of sink current from E1 to E2, perhaps by a downward clickof a joystick on an external controller as mentioned previously. At thispoint, the logic in the IPG 100 recognizes the need for sink current tobe simultaneously present at both E1 (8 mA) and E2 (2 mA). Accordingly,the logic in the IPG reconstructs these ideal pulses as shown in thesecond condition of FIG. 8A. This second condition reconfigures thepulses as fractionalized pulses. Because these pulses are fractionalizedand interleaved, their amplitudes are doubled to (in this example)approximately preserve the desired amount of charge. Thus, thefractionalized pulse portions at E1 (with a duty cycle of approximately50%), have an amplitude of approximately −16 mA over the same durationas the initial pulses, thus simulating the −8 mA pulse that is desired;the interleaved pulses portions at E2 likewise have an amplitude ofapproximately −4 mA to simulate the desired −2 mA pulse.

While the logic in the IPG 100 can be programmed to automaticallyfractionalize the pulses in a predetermined manner when necessary, itshould be noted that this is only an embodiment. The decision on how toperform the fractionalization can also be made by a user with a wirelessexternal controller. For example, a user can access a user interface onthe external controller to specify fractionalization parameters such asamplitude, duration, period, etc., with such parameters being wirelesslytransmitted to the nonvolatile memory storage in the IPG 100. Becauseexternal controllers are well known in the art, they are not discussedfurther.

Effecting such fractionalization is achieved in a preferred embodimentby re-writing the timing channel in which the initial condition wasspecified. In other words, moving from the initial to the secondcondition does not require the establishment of a second timing channel,because the logic in the IPG 100 preferably re-writes the first timingchannel to add the additional electrode (E2), to specify the duty cycleand period (t_(P)) of the first and second interleaved pulses, etc. Thisis an improvement over prior art current steering techniques whichrequire the use of steering using additional timing channels, such asthe '228 publication referenced earlier. In short, thepresently-disclosed current steering technique requires only a singletiming channel, making it amenable to IPG architectures having hardwareor software capable of handling only a single timing channel. Havingsaid this, it should be recognized that the invention can also beimplemented in IPGs having a plurality of timing channels, and so is notlimited to single timing channel devices.

As shown further in FIG. 8A, further user selection to steer anotherincrement of current results in decreasing the amplitude of thefractionalize pulse portions at the transmitting electrode E1 (from −16mA to −12 mA) while concurrently increasing the amplitude of thefractionized pulse portions at the receiving electrode E2 (from −4 mA to−8 mA). Such adjustment merely requires updating the amplitudes in thefirst (initial) timing channel, and does not require a second timingchannel. Continuing selections eventually result in the final desiredcondition illustrated at the bottom of FIG. 8B, in which the full amountof sink current (−10 mA) is now present entirety at electrode E2.Because this final condition does not require electrode E1 and E2 toboth sink current simultaneously, the pulses at E2 can be reconfiguredwith a 100% duty cycle pulse at its normal amplitude. Again, thishappens by re-writing the first timing channel.

It should be understood that reference to an “electrode on theimplantable stimulator device” includes electrodes on the implantablestimulator device, or the electrodes on the associated electrode leads,or any other structure for directly or indirectly stimulating tissue.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable medical device, comprising: sourcecircuitry configured to convert first stimulation parameters into acurrent that is sourced to a first electrode; sink circuitry configuredto convert second stimulation parameters into a current that is sunkfrom a second electrode; control logic configured to provide the firstand second stimulation parameters to the source and sink circuitry,wherein the first and second stimulation parameters define a group of aplurality of pulses, and wherein the plurality of pulses have a firstfrequency of greater than or equal to 2,000 Hz within the group, whereinthe first stimulation parameters define the group and wherein the secondstimulation parameters define a non-pulsed, constant-amplitude currentthat is simultaneous with the group.
 2. The implantable medical deviceof claim 1, wherein the first and second stimulation parameters define aperiodic application of the group and the simultaneous, non-pulsed,constant-amplitude current at a second frequency.
 3. The implantablemedical device of claim 2, wherein the periodic application of the groupand the simultaneous, non-pulsed, constant-amplitude current at thesecond frequency results in a time between consecutive applications ofthe group and the simultaneous, non-pulsed, constant-amplitude currentduring which no current is sourced or sunk.
 4. The implantable medicaldevice of claim 1, further comprising a receiver configured towirelessly receive the first and second stimulation parameters from adevice external to the implantable medical device.
 5. The implantablemedical device of claim 1, wherein the first and second stimulationparameters define a duration of the group.
 6. The implantable medicaldevice of claim 1, wherein the first and second stimulation parametersdefine the group within a single timing channel.
 7. The implantablemedical device of claim 1, wherein the pulses within the group have aduty cycle.
 8. The implantable medical device of claim 7, wherein theduty cycle is approximately 50%.
 9. The implantable medical device ofclaim 1, wherein the first frequency is less than or equal to 10,000 Hz.10. The implantable medical device of claim 1, wherein the sourcecircuitry and the sink circuitry comprise constant current circuitry.11. The implantable medical device of claim 1, further comprising firstand second leads coupled to the implantable medical device, wherein eachof the first and second electrodes is carried on one of the first andsecond leads.
 12. The implantable medical device of claim 11, whereinthe first and second leads are implanted on either side of aphysiological midline of a patient's spinal cord.
 13. The implantablemedical device of claim 11, wherein the first and second leads arepositioned within an epidural space of a patient's spinal cord.
 14. Theimplantable medical device of claim 1, wherein the current that issourced to the first electrode and the current that is sunk from thesecond electrode are equivalent in electrical charge.
 15. Theimplantable medical device of claim 1, wherein the source circuitry isfurther configured to convert third stimulation parameters into acurrent that is sourced to a third electrode.
 16. The implantablemedical device of claim 15, wherein the third stimulation parametersdefine a second group of a second plurality of pulses.
 17. Theimplantable medical device of claim 16, wherein the control logic isconfigured to provide the third stimulation parameters to the sourcecircuitry, and wherein the second plurality of pulses are interleavedwith the first plurality of pulses.
 18. A method, comprising: providing,with control circuitry in an implantable medical device, first andsecond stimulation parameters that define a group of a plurality ofpulses to source circuitry and sink circuitry within the implantablemedical device, wherein the plurality of pulses have a first frequencyof greater than or equal to 2,000 Hz within the group, and wherein thefirst stimulation parameters define the group and the second stimulationparameters define a non-pulsed, constant-amplitude current that issimultaneous with the group; converting, with the source circuitry, thefirst stimulation parameters into a current that is sourced to a firstelectrode; converting, with the sink circuitry, the second stimulationparameters into the constant-amplitude current that is sunk from asecond electrode.
 19. The method of claim 18, wherein the first andsecond stimulation parameters define a periodic application of the groupand the simultaneous, non-pulsed, constant-amplitude current at a secondfrequency.
 20. The method of claim 19, wherein the periodic applicationof the group and the simultaneous, non-pulsed, constant-amplitudecurrent at the second frequency results in a time between consecutiveapplications of the group and the simultaneous, non-pulsed,constant-amplitude current during which no current is sourced or sunk.